The goal is to develop an understanding of the biochemical mechanisms responsible for the metabolism of arachidonic acid and linoleic acid by these enzymes. Nitric oxide (NO) is reported to enhance prostaglandin formation by enhancement of PGHS activity. We found that NO is a substrate for the peroxidase of Cox-1 and-2 but it does not enhance the cyclooxygenase activity in vitro. In macrophages, NO does not stimulate the expression of Cox or alter LPS dependent Cox-2 expression. Furthermore, NO did not enhance prostaglandin formation in intact cells. Other studies with PGHS-1 indicate that a tyrosyl radical is formed which could be the reactive intermediate that initiates arachidonic acid oxygenation. Recently, we have shown that NO reacts with the tyrosyl radical converting the radical to a new radical characterized by ESR analysis as an iminoxyl radical. This leads to the formation of nitrotyrosine which is often used as an indicator of oxidative stress. We have also identified tyrosine 385 as the specific residue modified by NO which suggest that the tyrosine 385 is the source of the putative catalytically active tyrosyl radical.. Understanding the biochemical mechanisms which regulate the activities and expression of these enzymes will provide new insights into potentially controlling or preventing related disease conditions.